One of the fastest growing segments of the electro-optic art involves the use of photo electric detector arrays used in cameras and detectors for consumers, machine imaging and inspection imaging. The advancement is this area has been so extensive and so rapid at the technically advanced side of the market that the technology has enabled individually owned electronic cameras to begin to supplant cameras which use film and chemical development. The more technical side of the electronic imaging industry continues to advance and demands ever increasing sensitivity to produce a product of ever increasing quality. Increased affordability is had through mass production and the lowering of production costs while keeping the product quality high.
To consider a simple electronic camera as an example, the main component is a two dimensional electronic array, typically a silicon-based device having thousands of pixels of a size less than 20 micrometers each. The remaining parts of the camera are far less critical and include a lens, a focusing system for physically moving the lens, and computer memory storage. The quality and suitability of the two dimensional detector array will determine whether the camera will function properly. As it is the most expensive and critical component in the camera, if it is defective, the camera as a whole is virtually worthless.
The critical need is therefore to properly test two dimensional arrays with as much speed and accuracy as possible to eliminate the defective components very early in the manufacturing process, at each stage before additional value can be added. Quality control is of paramount importance in the products which use two dimensional detector arrays, but the even the tightest production and quality testing program cannot achieve its goals without the very most efficient test equipment. This problem is significant for low end products like ordinary digital cameras, but it is acute for high end and specialty two dimensional array products. Commercially available test illuminators do not provide uniform coverage over areas larger than about 24 square millimeters. There are not self-contained instruments for testing CCD arrays; such test instruments are generally assembled from separate components onto an optical bench, they are not suitable for the production environment, but rather for laboratory checks. As will be shown, currently available illumination test equipment fails to give the greatest efficiency both because of failure in spatial illumination and uniformity and because of losses in illumination intensity resulting in inefficiency. In addition, the structure gives greater ease of use.
Proper evaluation of the functional performance of large two dimensional detector arrays for camera vision requires spatially uniform levels of illumination. Commercially available test illuminators are low in efficiency, large, and bulky. Existing illuminators have achieved spatial uniformity approaching a one percent variance taken over a rather small its illuminating area. This value is unacceptable where high quality and very tight production control is essential. Existing illuminators are based upon the use of a spherical integrator to attempt to statistically randomize the distribution of illumination. A spherical integrator relies upon a relatively large sphere having an internal surface for re-radiating light directed toward the internal surface back onto an illumination test area. The idea behind the spherical illuminator is that even if the light source produces a partial shadow or a partial bright area as light emanates from the source, that using the spherical section to homogenize the light energy will "smooth out" the light or dark portion. This is based upon view factor, which uses the fact that each point in the illuminated field receives a fairly even summing contribution of the light from each portion of the spherical reflector. Where the target two dimensional array is placed so that it is surrounded by the hemispherical reflector, the maximum area (for illumination of one side of the two dimensional array) is available for each micro portion of the array to receive an even, summed average of the contribution from each re-radiating segment of the hemispherical array. Light from the lamp is spatially randomized by not only direct reflection from the lamp and the wall of the spherical section surface, but also by multiple internal sphere wall reflections which provide smoothing of severe light and dark spots. Usually, the internal spherical section is either made of a material having a very high, diffuse reflectance, and which can maintain the high, diffuse reflectance over a wide spectrum of light frequencies, or it has a surface coating of white paint possessing these similar light diffusing and handling properties.
Conventional reflecting spheres attempt to provide a uniform nearly ideal distribution of light, known as Lambertian distribution, where the reflected intensity is substantially independent of the angle of incidence. However, commercially available test illuminators are low in efficiency, large and bulky and do not provide uniform illumination coverage over the minimum required coverage area of about 24 millimeters unless they are very large. The output or reflective efficiency is a function of the overall area occupied by the radiating lamp, its base and other support structure, which may be referred to as a lamp port, the exit port of the sphere, and the whole wall area. The presence of ports is deleterious because it interferes with the homogenization process. The radiant output of an integrating sphere may be calculated from the following equation: EQU L.sub.out =R.phi..sub.1 /.pi.A[1-R(1-Ap/A)]
where R is the wall reflectance, .phi..sub.1 is the input flux, A is the sphere section wall area, and Ap is 2.0, the area of the ports.
As an example of the strong dependence on wall reflection, if the wall reflectance changes from about 0.98 to about 0.97, the value of L.sub.out changes to 86% of its former value. Thus the output is very critically sensitive to changes in reflectance. Wall reflectance must be precisely maintained over time since even the smallest change in the reflectance has a disproportionately large impact on efficiency.
The throughput efficiency is calculated for a sphere capable of illuminating a circle having four inch diameter. A reasonably sized sphere must be twelve inches in diameter; the input (lamp) port is about one inch in diameter. The energy throughput is calculated fro the following equation: EQU T+(R Ae/A)/(1-R[1-(Ae+Ai)/A]
Where Ae and Ai are the areas of the exit and entrance ports (which equal about 20.6 inches.sup.2 and where wall reflectance is about 0.98. This gives a throughput of about 42% of the available input energy.
The output distribution of an integrating sphere is uniform into about 2 .pi. steradians. The irradiance received at a detector to be tested is .OMEGA.N, where .OMEGA. is the solid angle subtended by the detector, and N is the radiance of the port. A detector to be tested and having an area of 1 centimeter by 1 centimeter square and located four inches from a seventy five millimeter diameter exit port will receive approximately 28% of the output of the sphere because it captures only 0.28 steradaiance. Multiplying the physical inefficiency of 0.28 times the transmission inefficiency of 0.42 gives an overall composite efficiency of about 0.12 or about 12%. In other words, and from a purely physical explanation, the amount of useable energy output from the sphere is only about 12% of the total available lamp power. Conversely, 88% of the energy is wasted.
Given this low level of efficiency, attempted compensation requires the use of a very high wattage lamp to power the illumination test system. A heating problem is thus created since about 80% of the energy going into the bulb is given off as waste heat which needs to be dissipated. Heat dissipation by providing openings in the sphere decrease would decrease its efficiency even further. A pure air ventilation system to compensate for the heat load would probably require refrigeration in order to work optimally. Resulting temperature changes from heating will introduce error into the two dimensional array measurement.
Further, the material from which the spherical wall is made is very expensive, regardless of whether it is machined from a solid or applied as a paint. The disadvantages are cost, large size and bulk and especially the waste heat energy which is not only a problem in itself, but as a source of error as stray light which unwantedly can heat the two dimensional array.
It is desirable to provide a relatively smaller beam cross section so that the homogeneity can be controlled. In the integrating sphere system it would be necessary to provide additional optics to reduce the beam diameter to a reasonable size to accommodate economical filter sizes, such as on the order of twenty five millimeters in diameter. The use of additional optics would further the already high losses of the integrating sphere and modify the solid angle factors of the optical system, thus reducing deliverable efficiency. The desirable small beam diameter is simply not practical with the integrating sphere. A structure is needed which is portable, efficient, stable compact and which facilitates field testing of imaging detector arrays and systems.